Since a physical leased line is expensive in spite of its superiority over a public network in security and quality of communication, a VPN (Virtual Private Network) has been widely used as a technique of constructing a virtual leased network on a public network.
In the present invention, the VPN refers to a technique of virtually realizing a leased line on a public network, which is a network shared by many unspecified users, by using a technique of warding off unauthorized access of third parties other than the intended users such as intrusion, wiretapping, interception, tampering, and spoofing.
That broadly-defined VPN includes the Internet VPN and the IP VPN.
Particularly the Internet VPN that uses the Internet for a transmission line has been rapidly widespread because it can be operated at a low cost and can be readily used in an Internet environment.
Hereinafter, the Internet VPN service that is currently widespread among the public will be referred to as the VPN service to distinguish it from the broadly-defined VPN.
Since the VPN service uses the Internet for the transmission line, it needs to manage authentication to detect a lack of authenticity of the VPN nodes and tampering with the VPN nodes between terminals in order to prevent unauthorized access of third parties other than the intended users such as wiretapping, tampering, or spoofing of the content of communications.
It also needs to encrypt packets by using an encryption key system and securely manage the key delivery to prevent leakage and tampering.
Now, typical methods of linking to the VPN service will be exemplified.
As a first method, a method of installing a VPN service program on a terminal of each user, first establishing communications among the terminals via specified global IP address information, then executing a VPN connection protocol can be considered.
In the first method, for specifying the destination's global IP address, a dynamic global IP address may be used through an external DNS (Domain Name System) service in addition to the way of setting a fixed global IP address to each terminal. In the first method, since end to end communication over the VPN service can be easily achieved but all of the exchange information required for the VPN link is transmitted through the Internet, the communication is vulnerable to an unauthorized access from outside. With user authentication and terminal authentication performed at the program level, this method is particularly vulnerable to spoofing attacks.
As a second method, a method of enhancing security by adding terminal authentication of each user can be considered. The second method is a method of first establishing communications among the terminals via specified global IP address information, then performing authentication to verify that the terminals are registered, and executing a VPN connection protocol.
The second method is a method of previously registering unique serial numbers of the terminals and crosschecking the numbers in establishing the VPN link to evaluate the authenticity of the VPN connection. Accordingly, the second method requires complicated work in managing registration and crosscheck of the unique serial number of each terminal and the like.
Since all the exchange information required for a VPN link is transmitted through the Internet also in this method as in the first method, the communication is vulnerable to unauthorized access from outside.
As a third method, a method of connecting the terminals of the respective users through carrier(s) that provide the external VPN service can be considered. The third method is a method of first connecting the terminals of the respective users with carrier(s) that provide the VPN service, then having the carrier(s) perform terminal authentication and user authentication and further perform acceptance of the destination information and a connection proxy service, and executing a VPN connection protocol.
In the third method, the user can use a VPN link securely and more easily than in the second method by previously installing a terminal registered with the carrier that is contracted with the user to provide the VPN service and establishing communication with the VPN service via the terminal to be subjected to terminal authentication and user authentication.
Since authentication of each terminal is individually performed by each carrier in the third method, exchange of destination information in connecting the terminals is simplified, therefore, the communication is less vulnerable to unauthorized access from outside than in the first and second methods.
However, the method requires the carriers which provide the VPN service to perform complicated setting of parameters and management for identifying and ensuring the VPN connection for each terminal, and the carriers in turn requires the user to bear the cost needed to maintain the service.
In addition, the use of the service will be limited in the area in which the environment for providing the VPN service is maintained by the carrier which provides the service.
Further, in view of the use of the VPN, techniques of delivering keys by using the Internet for a first network and using an audio circuit of fixed-line phone or mobile phone for a second network as disclosed in Patent Documents 1 and 2 have been proposed in order to enhance security and realize secure and fast key delivery between the VPN routers.
With the communication layered by using the second network of an audio circuit to deliver keys to be used in the confidential communication in the first network of a wide area network such as the Internet, secure and fast key delivery is expected.